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Ch.14 - Chemical Kinetics
All textbooksBrown 14th EditionCh.14 - Chemical KineticsProblem 76a
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Chapter 14, Problem 76a

(a) Most commercial heterogeneous catalysts are extremely finely divided solid materials. Why is particle size important?

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Surface Area to Volume Ratio

The surface area to volume ratio is crucial in catalysis because reactions occur at the surface of the catalyst. Smaller particles have a higher surface area relative to their volume, allowing more active sites for reactants to interact. This increased surface area enhances the catalyst's effectiveness, leading to faster reaction rates.
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Catalytic Activity

Catalytic activity refers to the ability of a catalyst to increase the rate of a chemical reaction without being consumed in the process. The particle size of a catalyst can influence its activity; smaller particles often exhibit higher activity due to more available active sites and better accessibility for reactants, which can lead to improved reaction efficiency.
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Diffusion Limitations

Diffusion limitations occur when reactants cannot reach the active sites of a catalyst quickly enough, which can hinder reaction rates. In heterogeneous catalysis, smaller particle sizes can reduce these limitations by allowing reactants to penetrate the catalyst more easily, thus facilitating faster reactions and improving overall catalytic performance.
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Related Practice
Textbook Question

You have studied the gas-phase oxidation of HBr by O2: 4 HBr(g) + O2(g) → 2 H2O(g) + 2 Br2(g)

You find the reaction to be first order with respect to HBr and first order with respect to O2. You propose the following mechanism:

HBr(g) + O2(g) → HOOBr(g)

HOOBr(g) + HBr(g) → 2 HOBr(g)

HOBr(g) + HBr(g) → H2O(g) + Br2(g)

(a) Confirm that the elementary reactions add to give the overall reaction.

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Textbook Question

You have studied the gas-phase oxidation of HBr by O2: 4 HBr(g) + O2(g) → 2 H2O(g) + 2 Br2(g)

You find the reaction to be first order with respect to HBr and first order with respect to O2. You propose the following mechanism:

HBr(g) + O2(g) → HOOBr(g)

HOOBr(g) + HBr(g) → 2 HOBr(g)

HOBr(g) + HBr(g) → H2O(g) + Br2(g)

(b) Based on the experimentally determined rate law, which step is rate determining?

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Textbook Question

(c) Do catalysts affect the overall enthalpy change for a reaction, the activation energy, or both?

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Textbook Question

The addition of NO accelerates the decomposition of N2O, possibly by the following mechanism: NO1g2 + N2O1g2¡N21g2 + NO21g2 2 NO21g2¡2 NO1g2 + O21g2 (b) Is NO serving as a catalyst or an intermediate in this reaction?

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Textbook Question

Many metallic catalysts, particularly the precious-metal ones, are often deposited as very thin films on a substance of high surface area per unit mass, such as alumina 1Al2O32 or silica 1SiO22. (b) How does the surface area affect the rate of reaction?

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Textbook Question

The enzyme urease catalyzes the reaction of urea, 1NH2CONH22, with water to produce carbon dioxide and ammonia. In water, without the enzyme, the reaction proceeds with a first-order rate constant of 4.15 * 10-5 s-1 at 100 C. In the presence of the enzyme in water, the reaction proceeds with a rate constant of 3.4 * 104 s-1 at 21 C. (c) In actuality, what would you expect for the rate of the catalyzed reaction at 100 C as compared to that at 21 C?

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